biomolecules Review Regulation of Tissue Inflammation by 12-Lipoxygenases Abhishek Kulkarni 1 , Jerry L. Nadler 2, Raghavendra G. Mirmira 1,* and Isabel Casimiro 1,* 1 Department of Medicine, The University of Chicago, Chicago, IL 60637, USA; [email protected] 2 Department of Medicine and Pharmacology, New York Medical College, Valhalla, NY 10595, USA; [email protected] * Correspondence: [email protected] (R.G.M.); [email protected] (I.C.) Abstract: Lipoxygenases (LOXs) are lipid metabolizing enzymes that catalyze the di-oxygenation of polyunsaturated fatty acids to generate active eicosanoid products. 12-lipoxygenases (12-LOXs) primarily oxygenate the 12th carbon of its substrates. Many studies have demonstrated that 12-LOXs and their eicosanoid metabolite 12-hydroxyeicosatetraenoate (12-HETE), have significant pathological implications in inflammatory diseases. Increased level of 12-LOX activity promotes stress (both oxidative and endoplasmic reticulum)-mediated inflammation, leading to damage in these tissues. 12-LOXs are also associated with enhanced cellular migration of immune cells—a characteristic of several metabolic and autoimmune disorders. Genetic depletion or pharmacological inhibition of the enzyme in animal models of various diseases has shown to be protective against disease development and/or progression in animal models in the setting of diabetes, pulmonary, cardiovascular, and metabolic disease, suggesting a translational potential of targeting the enzyme for the treatment of several disorders. In this article, we review the role of 12-LOXs in the pathogenesis of several diseases in which chronic inflammation plays an underlying role. Citation: Kulkarni, A.; Nadler, J.L.; Keywords: 12-lipoxygenases; 12-LOXs; 12/15-lipoxygenase; 12/15-LOX; lipoxygenases; eicosanoids; Mirmira, R.G.; Casimiro, I. inflammation Regulation of Tissue Inflammation by 12-Lipoxygenases. Biomolecules 2021, 11, 717. https://doi.org/10.3390/ biom11050717 1. Introduction Inflammation is a conserved mechanism that serves as a defense against injurious Academic Editors: George Kokotos stimuli, including invasion of pathogens, tissue injury, and intracellular damage signals [1]. and Sasanka Ramanadham Cells release a variety of factors, including histamines, prostaglandins, and bradykinin. These signals promote an acute inflammatory response, including changes to vascular per- Received: 30 March 2021 meability, with localized infiltration and accumulation of immune cells from the circulation, Accepted: 7 May 2021 Published: 11 May 2021 accompanied by the release of inflammatory mediators such as cytokines, chemokines, and eicosanoids. This cascade of inflammation is followed by tissue remodeling and a repair process to restore tissue health. Usually, this inflammatory response is completed Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in upon the eradication of the pathogens or after tissue repair. However, if the process is not published maps and institutional affil- appropriately terminated or becomes uncontrolled, it can lead to a maladaptive chronic iations. inflammatory state that contributes to irreversible tissue damage resulting in disease pathol- ogy. Chronic inflammation is known to be an underlying cause of several diseases, such as metabolic syndrome, type 1 and type 2 diabetes (T1D and T2D), non-alcoholic fatty liver disease (NAFLD), hypertension, cardiovascular disease (CVD), chronic kidney disease, neurodegenerative, and autoimmune diseases [2]. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. 2. The Mammalian Lipoxygenases This article is an open access article distributed under the terms and Lipoxygenases (LOXs) collectively represent a family of enzymes that catalyze the conditions of the Creative Commons oxygenation of cellular polyunsaturated fatty acids (PUFAs) to form eicosanoid metabo- Attribution (CC BY) license (https:// lites that function in inflammatory pathways in an autocrine, paracrine, or endocrine creativecommons.org/licenses/by/ fashion [3,4]. The LOX enzymes catalyze the oxidative metabolism of multiple PUFA sub- 4.0/). strates, including arachidonic acid (AA), dihomo-γ-linoleic acid (DLA), linolenic acid (LA), Biomolecules 2021, 11, 717. https://doi.org/10.3390/biom11050717 https://www.mdpi.com/journal/biomolecules Biomolecules 2021, 11, x FOR PEER REVIEW 2 of 19 Biomolecules 2021, 11, 717 ion [3,4]. The LOX enzymes catalyze the oxidative metabolism of multiple PUFA sub-2 of 18 strates, including arachidonic acid (AA), dihomo-γ-linoleic acid (DLA), linolenic acid (LA), docosahexaenoic acid (DHA), and eicosapentaenoic acid (EPA) to generate a multi- tudedocosahexaenoic of bioactive products acid (DHA), involved and eicosapentaenoicin pro- and anti-inflammatory acid (EPA) to ac generatetivities.a The multitude cellular of eventsbioactive that productsoccur during involved resolution in pro- of and inflammation anti-inflammatory employ activities. a conversion The cellularfrom pro events-in- flammatorythat occur duringmetabolites resolution to pro of-resolving inflammation mediators, employ such a conversion as lipoxins from and pro-inflammatory resolvins, to dampenmetabolites the immune to pro-resolving response (Figure mediators, 1) [5] such. Arachidonic as lipoxins acid and serves resolvins, as a major to dampen precursor the forimmune a variety response of eicosanoids (Figure 1with)[ 5 ].inflammatory Arachidonic properties acid serves including as a major the precursor product for 12- aHETE, variety whichof eicosanoids will be the with focus inflammatory of this review properties. including the product 12-HETE, which will be the focus of this review. Figure 1. Substrates and products of the 12-lipoxygenases. Figure 1. Substrates and products of the 12-lipoxygenases. LOXs are expressed in all cell types of hematopoietic origin, in particular, platelets and leukocytes.LOXs are Their expressed nomenclature in all cell is types based of on hematopoietic the location atorigin, which in they particular insert, the platelets oxygen andon leukocytes. their fatty acid Their substrate nomenclature [6,7]. 12-LOX is based catalyzes on the location the oxygenation at which they of the insert 12th t carbonhe oxy- of genarachidonic on their fatty acid acid to 12-hydroperoxyeicosatetraenoate substrate [6,7]. 12-LOX catalyzes the (12(S)-HPETE), oxygenation whichof the 1 subsequently2th carbon ofis arachidonic reduced by glutathioneacid to 12-hydroperoxyeicosatetraenoate peroxidase to a more stable analogous (12(S)-HPETE), hydroxy which compound subse- 12- quentlyhydroxyeicosatetraenoate is reduced by glutathione 12(S)-HETE, peroxidase or simply to a 12-HETEmore stable [8–10 analogous]. In mice, hydroxy there are com- seven poundfunctional 12-hydroxyeicosatetraenoate LOX genes (Alox5, Alox12, 12(S) Alox12b,-HETE Alox15,, or simply Alox8, 12- Aloxe3,HETE [8and–10].Aloxe12 In mice,), three there or arefour seven functional functional genes LOX in zebrafish genes ( (Alox5,alox5a, Alox12, alox5b, alox12Alox12b,, and Alox15, a possible Aloxalox158, Aloxe3,orthologue) and Aloxe12and six), functionalthree or four genes functional in humans genes (ALOX5, in zebrafish ALOX12, (alox5a, ALOX12B, alox5b, ALOX15, alox12, and ALOX15B, a possibleand alox15ALOXE3 orthologue)). Their expression and six functional patterns vary genes by in tissue humans distribution (ALOX5, and ALOX12, cell type. ALOX12B, ALOX15,The ALOX15B, genomic distribution and ALOXE3 of). the Their LOX expression genes shows patterns conservation vary by across tissue higherdistribution species. andAll cellAlox typgenese. in mice, except for Alox5, are located in the LOX cluster 1 and 2 of chromo- someThe 11 genomic [6]. Notably, distribution these lipoxygenase of the LOX genes clusters shows reside conservation near other chemokine across higher clusters, species. and Allthe Alox genetic genes proximity in mice, except is conserved for Alox in5 humans., are located Whereas in theALOX5 LOX clusteris located 1 and on 2 chromosomeof chromo- some10 in 11 humans, [6]. Notably the rest, these of the lipoxygenase LOX genes areclusters clustered reside in near chromosome other chemokine 17p13.1 nearclusters, other andgenes the genetic involved proximity in inflammation is conserved [11,12 in]. humans. Whereas ALOX5 is located on chromo- some 10 in humans, the rest of the LOX genes are clustered in chromosome 17p13.1 near other3. The genes 12/15-Lipoxygenase involved in inflammation Pathway [11,12]. 3.1. Functional Orthologs of 12-LOX Human LOX enzymes include 5-lipoxygenase (5-LOX), which result in the production of leukotrienes, as well as the 12-lipoxygenases and the 15-lipoxygenases. 12-lipoxygenase was the first identified mammalian lipoxygenase. It was discovered in human and bovine platelets in the mid-1970s [13–15]. Different isoforms of 12-lipoxygenase have been identi- fied and named after the cells in which they were found: platelet, leukocyte, and epidermal Biomolecules 2021, 11, 717 3 of 18 type. However, this convention can be problematic as it is not always precise. For instance, the “platelet type 12-LOX” can be found in both platelets and skin of humans and mice, but is not expressed in porcine platelets [15,16]. To add more complexity to the nomen- clature, there is significant species-specific variation in terms of the products formed by 12-LOX and 15-LOX. In this regard, the use of the appropriate nomenclature is imperative, as orthologs of the same gene may have different stereo-specificities